This invention relates to a process for transferring a thin film of solid material. In particular, this process can be used to transfer a thin film of solid material onto a support composed of a solid material of the same nature or a different nature.
Document FR-A-2 681 472 (corresponding to patent U.S. Pat. No. 5, 374, 564) describes a process for making thin films of semiconducting material. This document divulges that the implantation of a rare gas or hydrogen into a substrate made of a semiconducting material can cause the formation of a layer of micro-cavities or micro-bubbles (also denoted xe2x80x9cplateletsxe2x80x9d) at a depth close to the average projected range (Rp) of the implanted ions. The concept of micro-cavities obviously includes micro-cracks. The thickness of the layer of micro-cavities is determined by the implantation conditions. If this substrate is put into intimate contact with a stiffener through its implanted face and a heat treatment is applied at a sufficiently high temperature, an interaction occurs between the micro-cavities or the micro-bubbles separating the semiconducting substrate into two parts, firstly a thin semiconducting film bonding to the stiffener, and secondly the remainder that bonds to the semiconducting substrate. Separation takes place at the location of the micro-cavities or micro-bubbles. The heat treatment is such that the interaction between the micro-bubbles or micro-cavities created by the implantation induces a separation between the thin film and the remainder of the substrate. Therefore a thin film is transferred from an initial substrate to a stiffener used as a support for this thin film.
This process can also be applied to the manufacture of a thin film of a crystalline or non-crystalline solid material other than a semiconducting material (a conducting or dielectric material).
If the thin film delimited in the substrate is sufficiently stiff in itself (due to its thickness or due to its mechanical properties) a self-supported film may be obtained after the transfer annealing. This is described in document FR-A-2 738 671.
Document EP-A-0 767 486 proposes an improvement to the process divulged in document FR-A-2 681 472 mentioned above. According to document EP-A-0 767 486 (see column 8), the process divulged by document FR-A-2 681 472 has the following disadvantages. The choice of the thickness of the film to be transferred is a weak degree of freedom. The thickness of the film to be transferred (corresponding to Rp) and the conditions for separation of the film from the initial substrate are inter-related. The planeness of the film surface obtained after separation is unsatisfactory, and there is no way of maintaining a uniform thickness of a thin film during the transfer. The improvement proposed by document EP-A-0 767 486 consists of performing the ion implantation at depth Rp in a porous layer of silicon formed on the surface of a silicon substrate. This ion implantation causes an increase in the porosity (pore density) to the extent that micro-cavities appear in the walls of the pores of the porous layer. This layer is then considered as being a fine porous structure. Under some implantation conditions, separation is caused in this fine porous layer in accordance with the mechanism described in document FR-A-2 681 472. Therefore, there are two zone effects, firstly due to a zone of pores created by a porous silicon generation step, and secondly due to a zone of cavities generated between the pores in the small perfect silicon zones as in the process according to document FR-A-2 681 472. Therefore, the proposed improvement consists of using a porous layer to obtain a layer with a well-controlled uniform thickness after separation.
The process divulged by document EP-A-0 767 486 recommends the formation of porous silicon (the order of the porosity is a percentage equal to several tens), which is equivalent to removing silicon or material from the separation zone which causes weakening of the material.
A more significant improvement to the process revealed by document FR-A-2 681 472 would be to reduce thickness of the micro-cavities layer obtained by ion implantation. This is what is proposed in this invention.
The improvement proposed by this invention is made possible due to creation of an inclusion or a set 6f inclusions in the initial substrate material, in order to confine gaseous compounds introduced during the ion implantation step. An inclusion is a volume of material for which the properties are not the same as the properties of the substrate material from which one or more thin films are to be transferred. Inclusions may be in the form of a layer that extends approximately parallel to the surface through which the implantation is done. These volumes may have a variety of shapes and their dimensions may vary from a few tens of nanometers to several hundreds of micrometers.
The role of these inclusions is to act as traps for implanted gaseous compounds. The radius of action of these traps depends on the nature of the inclusions made. In this case, there is no removed material, as is the case for the process divulged by document EP-A-0 767 486.
The process according to this invention comprises a preliminary step that consists of forming inclusions in the initial substrate material. A subsequent step consists of implanting gaseous compounds (rare gas or other) in this material. The presence of inclusions formed during the previous step causes confinement of implanted gaseous compounds. The efficiency of these inclusions is related to their power to confine gaseous compounds.
Inclusions may be formed close to a perfectly controllable depth. Their presence then introduces confinement of implanted compounds within a disturbed layer which is much thinner than can be obtained using the process according to known art. This produces several advantages. The implanted gaseous compounds are preferably trapped at the level and/or within the zone influenced by these inclusions, called the neighborhood of these inclusions. This precise position means that a separation (transfer) fracture can be induced at and/or near the inclusions. The result is a relatively low surface roughness at the fracture. Furthermore, due to the confinement power, this process enables the use of low implanted doses necessary for the fracture. Finally, the confinement effect due to the presence of inclusions can reduce the thermal budget necessary for the fracture, to the extent that nucleation and growth of cavities leading to fracture is encouraged. The advantage is obvious for transferring film structures in which there is a limit on the maximum temperature rise. For example, one case is the heterogeneous gluing of materials with coefficients of expansion that differ by more than 10%.
Therefore, the purpose of the invention is a process for the transfer of at least one thin film of solid material delimited in an initial substrate, characterized in that it comprises the following steps:
a step in which a layer of inclusions is formed in the initial substrate at a depth corresponding to the required thickness of the thin film, these inclusions being designed to form traps for gaseous compounds which will subsequently be implanted;
a subsequent step for implantation of the said gaseous compounds, in a manner to convey the gaseous compounds into the layer of inclusions, the dose of implanted gaseous compounds being sufficient to cause the formation of micro-cavities likely to form a fracture plane along which the thin film can be separated from the remainder of the substrate.
The step of implanting gaseous compounds can be carried out with an implantation energy of these gaseous compounds that is such that their mean depth of penetration into the substrate corresponds to the depth of the layer of inclusions. It can also be carried out with an implantation energy of these gaseous compounds that is such that their mean depth of penetration into the substrate is close to the layer of inclusions, this implantation being associated with a diffusion heat treatment to allow the migration of the implanted compounds to the layer of inclusions.
The implantation step may be performed from one or several gaseous compounds implanted either simultaneously or in sequence.
The initial substrate may be composed of a solid part supporting a structure composed of one or more films, in which the said film of solid material must be delimited. All or part of this structure may be obtained by epitaxy. This structure may be such that the remainder of the substrate, which may or may not be an epitaxy carrier, can be reused after the thin film has been transferred to transfer another thin film.
The layer of inclusions may be formed by a film deposition technique. It may then consist of generating columns or generating grains.
Inclusions may have a chemical affinity with the said gaseous compounds.
Inclusions may originate from a parametric mismatch between the material forming the inclusions layer and substrate regions adjacent to it. This parametric mismatch may consist of a change in the size of crystalline parameters, changes in the crystalline orientation along a plane parallel to the surface of the transferred structure, a difference in the coefficient of thermal expansion between one of the films and the initial material (and/or other films).
The inclusions layer may also be formed by a technique for etching a substrate layer.
It may also be formed by the implantation of elements in a substrate layer. These elements may be implanted in one or several steps. Implantation of these elements may be assisted by heat treatment capable of increasing the efficiency of traps, this heat treatment possibly being done before, during and/or after implantation. This heat treatment may modify the morphology and/or composition of the inclusions, which encourages subsequent confinement of gaseous compounds. This heat treatment is done at a temperature and for a period such that it cannot be used to make a fracture over the entire inclusions layer.
The inclusions layer may also be obtained by heat treatment of the film(s) and/or by applying stresses to the film(s) in the film structure.
The inclusions layer may also be obtained by a combination of the different techniques mentioned above.
The gaseous compounds may be implanted by bombardment of the compounds chosen among neutral compounds and ions. It may also be done by a method chosen from plasma assisted diffusion, thermal diffusion and plasma assisted diffusion combined with thermal diffusion and/or assisted by electric polarization. Implantation may take place normal to the implanted surface of the substrate, or at a certain incidence. It may be done using rare gases, or other elements.
The process may comprise a heat treatment step capable of weakening the substrate at the inclusions layer to enable separation between the thin film and the remainder of the substrate. This heat treatment is applied with a given thermal budget which depends on the various thermal budgets used during the process. In particular, this heat treatment takes account of the temperature rise(s) induced by heat treatments in which thermodynamic equilibrium is not achieved, such as temperature rises resulting from the inclusions formation step and/or the step of implanting gaseous compounds and heat treatments involving heating or cooling of the substrate, for example such as for implantation, or reinforcement of the bond forces when gluing on a support. Therefore this heat treatment may be zero if the said weakening can be achieved by other steps in the process. It may be achieved by applying a positive temperature or a negative temperature. This weakening according to the invention is such that it enables separation of the thin film from the remainder of the substrate with or without the use of mechanical stresses. This heat treatment may be obtained by pulsed heating, for example in order to quickly increase the temperature. For example, this pulsed heating may be of the RTA (Rapid Thermal Annealing) or RTP (Rapid Thermal Process) type.
The process may also comprise a step in which the thin film delimited in the substrate is put into intimate contact with a support onto which the thin film will bond after it has separated from the remainder of the substrate. The film may be put into intimate contact directly (for example by wafer bonding) or through an added on material. A heat treatment step may be used to reinforce the bond between the thin film delimited in the substrate and the added on support.
Mechanical stresses may be exerted during and/or after and/or before the heat treatment, to contribute to separation between the thin film and the remainder of the substrate.
The process according to the invention is particularly suitable for the transfer of a thin silicon film from an initial substrate. It may also be applied for the transfer of a thin film made of a III-V semiconducting material (for example AsGa), from an initial substrate. The thin film may itself be composed of a thin film structure. It may have been at least partially treated before its transfer, to form, over all or part of the film to be transferred, an integrated circuit or to form, over all or part of the film to be transferred, an optoelectronic component on it.